The ability of cells to respond to environmental cues is in large part due to the interaction of cell-surface receptors with external stimuli. A number of factors that interact with receptors have been identified, among which are receptor-binding proteins that are soluble, membrane-bound, or exist in both forms.
One class of receptors, the receptor tyrosine kinases (RTKs), has been intensively studied and shown to be crucial to the growth and differentiation of a variety of cell types (Yarden and Ullrich, Ann. Rev. Biochem. 57:433-478, 1988). Tyrosine kinases are enzymes that catalyze the phosphorylation of tyrosine residues. Tyrosine phosphorylation is associated with signal-transduction across the cellular plasma membrane. The protein-tyrosine kinase family can be grouped into two very broad families: the above-mentioned RTKs, which are, or are intimately associated with, membrane-spanning growth factor receptors; and those that are associated with the membrane but lack a transmembrane sequence (Yarden and Ullrich, 1988, supra.).
The RTKs can be further divided into five subgroups on the basis of structural similarities in their extracellular domains and the organization of the tyro sine kinase catalytic region in their cytoplasmic domains. Subgroups I (epidermal growth factor (EGF) receptor-like), II (insulin receptor-like) and the eph/eck family contain cysteine-rich sequences (Hirai et al., Science 238:1717-1720, 1987; Yanden and Ullrich, Ann. Rev. Biochem, 57,443-478, 1988; Lindberg and Hunter, Mol. Cell. Biol.. 10:6316-6324, 1990). The functional domains of the kinase region of these three classes of RTKs are encoded as a contiguous sequence (Hanks et al., Science 241, 42-52, 1988). Subgroups III (platelet-derived growth factor (PDGF) receptor-like) and IV (the fibroblast growth factor (FGF) receptors) are characterized as having immunoglobulin (Ig)-like folds in their extracellular domains, as well as having their kinase domains divided in two parts by a variable stretch of unrelated amino acids (Yanden and Ullrich, 1988; Hanks et al., 1988).
While all members of the RTK family share a related cytoplasmic catalytic domain, the extracellular, ligand-binding domains of these receptors have adapted patchwork structure utilizing several structural motifs. The variablility in the structure of the ligand-binding domains of the RTKs almost certainly reflects the diversity of the ligands for these receptors (Ullrich and Schlessinger, Cell 61,203-212, 1990). These ligands range from relatively small, soluble peptides to cell surface proteins that themselves resemble receptors. Examples of ligands bound by certain members of the RTK family are polypeptide growth factors and hormones.
To deal with this diversity of ligands the RTKs have evolved extracellular domains that are a composite of several structural motifs. For example, the extracellular domain of the axl/ark gene contains both Ig-domains and fibronectin type III (FNIII) repeats (O'bryan et al., Mol. Cell. Biol. 11:5016-5031, 1991; Rescigno et al., Oncogene 6:1909-1913, 1991), while members of the eph family have those two motifs separated by a (non-EGF-like) cysteine-rich domain (Hirai et al., Science 238:17 17-1720, 1987; Lindberg et al., Mol. Cell. Biol. 10:6316-6324, 1990; Lhotak et al., Mol. Cell. Biol. 11:2496-2502, 1991; Chan and Watt, Oncogene 6:1057-1061,1991). This diversity strongly suggests that this family of receptors evolved by accumulating the structural motifs needed for ligand binding and combining these motifs with a conserved catalytic don-min.
In view of the role tyrosine kinases play in cell growth and differentiation, as well as signal transduction, isolation of novel tyrosine kinases enables one to study such biological processes. Identifying tyrosine kinases and their ligands also permits exploration of methods for inhibiting or enhancing the interaction thereof, depending on the desired biological effect.